Lightning Activity over Large Urban Areas of the Southeastern Brazil

This paper presents the preliminary results from CG lightning characteristics analysis over three large urban areas in southeastern Brazil: the São Paulo metropolitan area, Vale do Paraíba Paulista and the Campinas metropolitan area. A lightning dataset of about 310,000 flashes was obtained by the Brazilian lightning detection network during the months of December, January and February from 1999 to 2002 (three summer seasons). It was observed an enhancement of the CG lightning density over the urban areas (specially in Sao Paulo city, with values of 7-8 flashes.km.summer) with a decrease in the percentage of positive flashes. PM10 concentrations were also compared with the lightning frequency in seven areas of 10km radius including the measurement stations. It was found a linear correlation between the PM10 measurements and the related lightning activity. Furthermore, the spatial distribution of CG flashes follows closely the shape of the heat island in São Paulo city. These results support the need of more detailed investigation to assess which is the prevailing effect on the enhancement of the lightning activity: the urban heat island (thermal hypothesis) or the anthropogenic particulate emissions (aerosol hypothesis). INTRODUCTION The harmful effects of lightning flashes over highly populated areas have motivated several studies on lightning activity over these regions. Westcott [1995] was the first to show an enhancement of the cloud-toground (CG) lightning density over large cities in the USA using data from the National Lightning Detection Network (NLDN). She considered a combination of several factors, including the increase of cloud condensation nuclei (CCN) concentrations, and the urban heat island, as an explanation to her findings. Orville et al. [2001], using a twelve-year period (1989-2000) NLDN data, found an enhancement of the lightning density over Houston (TX). They also explained the elevated flash densities by means of several factors working together: an increase of convergence due to the heat island effect and the high concentrations of CCN. Pablo and Soriano [2002] found that not only large cities could contribute to the enhancement of CG lightning activity. They studied nine small cities in Spain using a 3-year lightning network data and found increasing in CG lightning flash density. Using PM10 and SO2 concentrations as gross indicators of CCN, they suggested a link between the SO2 concentration and the variation in the number of flashes. However, no correlation between the lightning frequency and the PM10 concentration was found. In a very comprehensive work, Steiger et al. [2002] showed the effect of the Houston urban area not only on the CG flash densities but also on the percentage of positive flashes, which decreases as the number of CG flashes increases. In Brazil, Gomes [2002] found a similar result over Belo Horizonte metropolitan area using lightning network data from an 8-year period (1988-1996). Steiger et al. [2002] proposed many factors to explain their findings, without point out a primary reason: the influence of the CCN on the cloud droplet spectra, an increase of the convergence over the urban area, and the effect of impurities in cloud water on the altitude of the negative charge region in thunderclouds.